This invention relates to a guidance system for a self-guided vehicle. More particularly, it relates to a system for providing position information to the vehicle and for controlling certain aspects of the vehicle's operation in response thereto, such as precision stopping at predetermined locations as the vehicle travels along a predetermined path.
Many self-guided vehicles used in industrial applications such as warehouses and the like are of the type capable of following a path defined by a current-carrying guide wire arranged on, or more typically embedded in a groove in, the surface over which the vehicle travels. Vehicles of this type, such as those disclosed in Kohls U.S. Pat. No. 3,411,603, Thompson et al. U.S. Pat. No. 3,768,586, Schnaibel U.S. Pat. No. 4,247,896, Nishiki et al. U.S. Pat. No. 4,456,088, Tax et al. U.S. Pat. No. 3,669,206 and Taylor U.S. Pat. No. 4,307,329, generally employ induction coils to sense the magnetic field caused by the AC current in the guide wire, and use steering control signals generated by the coils to detect and correct any deviation from the guide wire so as to guide the vehicle along the path defined by the guide wire.
In addition to the steering reference signal provided by the magnetic field of the guide wire, other control information, such as vehicle position along the path of travel and the location of destinations where the vehicle is to stop and pick up or deliver cargo, is needed. Some self-guided vehicle systems, such as that disclosed by Tax et al., employ sensing loops along the pathway for sensing the approximate position of the vehicle as it passes by the sensing loops. Other systems, such as that disclosed by Thompson et al., employ a sensor unit on the vehicle to sense approximate position data contained in nodes or loops formed by the guide wire(s). Still other systems, such as those disclosed by Kohls, and by Uemura U.S. Pat. No. 3,653,456, employ magnets, placed alongside the path and spaced transversely from the guide wire, to provide approximate position information to sensors mounted on the vehicle.
A major installation problem associated with systems employing a plurality of magnets spaced from the conductor, such as disclosed in Kohls, is that the magnets must be embedded in the surface over which the vehicle travels at locations spaced transversely from the guide wire, requiring a plurality of holes to be formed in such surface which interrupt the integrity of the surface and raise the installation cost of the system. Although it would be much less costly and less disruptive to the surface to embed the magnets in the same groove which contains the embedded guide wire, this has not been considered possible because the magnets would distort the magnetic field of the wire both by shunting the field away from the sensing coils on the vehicle and by creating interfering induced eddy current fields. Similarly, the guide wire node or loop systems described above would also result in a substantial installation expense and disruption of the supporting surface. An additional disadvantage to a system such as disclosed in Kohls is that an array of magnets arranged transversely to the direction of vehicle travel requires a corresponding array of sensors on the vehicle.
Because the foregoing systems provide only approximate position information in any case, most of the self-guided vehicles of the type described above must employ fifth wheel encoders to provide more precise vehicle position information by recording travel distances to enable the vehicle to be stopped at precise positions for loading and unloading cargo. For high accuracy, however, it is necessary continually to correct or update such encoders to compensate for slip, wheel wear, or floor irregularities which cause erroneous distance readings. Accordingly, another problem associated with such self-guided vehicles is that of providing highly accurate and reliable position signals which will enable the vehicle to decelerate and stop precisely at prescribed locations.